U.S. patent application number 13/612548 was filed with the patent office on 2013-01-03 for method for predetermined component placement to a target platform.
This patent application is currently assigned to Wintec Industries, Inc.. Invention is credited to Kong-Chen Chen.
Application Number | 20130000104 13/612548 |
Document ID | / |
Family ID | 46205864 |
Filed Date | 2013-01-03 |
United States Patent
Application |
20130000104 |
Kind Code |
A1 |
Chen; Kong-Chen |
January 3, 2013 |
Method for Predetermined Component Placement to a Target
Platform
Abstract
Method for aligning electrical contacts of first and second
electronic circuits or assemblies, the first and second electronic
circuits or assemblies each being provided with alignment features.
The method includes placing electrical contacts of the second
electronic circuit or assembly in contact or near contact with
electrical contacts of the first electronic circuit or assembly. An
alignment condition of alignment features of the first and second
electronic circuits or assemblies are electrically sensed during
the contact or near contact to obtain sensing information. The
sensing information is stored and analyzed to determine a movement
to be caused between the first and second electronic circuits or
assemblies to achieve an improved alignment condition. Each of the
alignment features comprises a plurality of alignment elements. The
electrically sensing step electrically senses an alignment
condition of alignment elements of alignment features of the first
and second electronic circuits or assemblies.
Inventors: |
Chen; Kong-Chen; (San Jose,
CA) |
Assignee: |
Wintec Industries, Inc.
Milpitas
CA
|
Family ID: |
46205864 |
Appl. No.: |
13/612548 |
Filed: |
September 12, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12779047 |
May 13, 2010 |
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13612548 |
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11351418 |
Feb 10, 2006 |
7928591 |
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12779047 |
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60652217 |
Feb 11, 2005 |
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Current U.S.
Class: |
29/593 |
Current CPC
Class: |
H01L 2924/01082
20130101; H01L 2224/81801 20130101; H01L 2924/01075 20130101; H01L
2924/19041 20130101; H01L 2924/01038 20130101; H01L 2924/01006
20130101; H01L 2924/19043 20130101; H01L 2924/01033 20130101; H01L
24/81 20130101; H01L 2223/5444 20130101; H01L 2224/16 20130101;
H01L 2924/01005 20130101; H01L 2224/81121 20130101; H01L 2924/01023
20130101; Y10T 29/49004 20150115; H01L 2924/30105 20130101; Y10T
29/53261 20150115; H01L 2924/01013 20130101; H01L 2924/14 20130101;
H01L 2924/12042 20130101; H01L 2924/12042 20130101; Y10T 29/49778
20150115; H01L 23/544 20130101; H01L 2924/12041 20130101; H01L
2924/00 20130101; H01L 2924/351 20130101; H01L 2924/014 20130101;
H01L 22/32 20130101; H01L 2223/5448 20130101 |
Class at
Publication: |
29/593 |
International
Class: |
G01R 31/28 20060101
G01R031/28 |
Claims
1. A method for aligning electrical contacts of a second electronic
circuit or assembly with electrical contacts of a first electronic
circuit or assembly, the first electronic circuit or assembly and
the second electronic circuit or assembly each being provided with
alignment features, the method comprising: placing electrical
contacts of the second electronic circuit or assembly in contact or
near contact with electrical contacts of the first electronic
circuit or assembly; electrically sensing an alignment condition of
alignment features of the second electronic circuit or assembly and
alignment features of the first electronic circuit or assembly
during the contact or near contact to obtain a sensing information;
storing the sensing information; and analyzing the sensing
information to determine a movement to be caused between the second
electronic circuit or assembly and the first electronic circuit or
assembly to achieve an improved alignment condition, wherein each
of the alignment features comprises a plurality of alignment
elements, and wherein the electrically sensing step electrically
senses an alignment condition of alignment elements of an alignment
feature of the second electronic circuit or assembly and alignment
elements of an alignment feature of the first electronic circuit or
assembly.
2. The method of claim 1 including moving the second electronic
circuit or assembly to achieve an improved alignment condition
following the electrically sensing.
3. The method of claim 1, wherein the electrically sensing
includes: providing a probe arrangement having a plurality of
sensor elements to electrically sense an alignment condition; and
dynamically configuring the probe arrangement to select a subset of
the sensor elements for sensing the alignment condition.
4. The method of claim 1 including: providing a probe arrangement
to electrically sense an alignment condition; holding the first
electronic circuit or assembly stationary; and aligning the probe
arrangement in relation to the first electronic circuit or
assembly.
5. The method of claim 1 wherein the electrically sensing includes
applying an electrical stimulus to the alignment feature to
electrically sense presence or absence of the electrical
stimulus.
6. The method of claim 5 wherein the sensing is resistive.
7. The method of claim 5 wherein the sensing is capacitive.
8. The method of claim 5 including applying an electrical stimulus
in parallel to a plurality of alignment features.
9. The method of claim 5 including applying an electrical stimulus
in series to a plurality of alignment features.
10. The method of claim 1 including pre-storing dimensional
information pertaining to an alignment feature prior to
electrically sensing.
Description
CROSS-REFERENCES TO RELATED APPLICATION
[0001] This application is a continuation of commonly assigned U.S.
Non-Provisional application Ser. No. 12/779,047 titled "Apparatus
and Method for Predetermined Component Placement to a Target
Apparatus" filed May 13, 2010, which is a continuation of commonly
assigned U.S. Non-Provisional application Ser. No. 11/351,418 of
the same title filed Feb. 10, 2006, now U.S. Pat. No. 7,928,591,
which claims priority to commonly assigned U.S. Provisional
Application No. 60/652,217 filed Feb. 11, 2005, the contents of all
of which are incorporated herein by reference in their
entirety.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to assembly
techniques. The alignment and probing techniques to improve the
accuracy of component placement in assembly are described. More
particularly, the invention includes methods and structures to
detect and improve the component placement accuracy on a target
platform by incorporating alignment marks on component and
reference marks on target platform with various probing techniques.
A set of sensors grouped in an array to form a multiple-sensor
probe can detect the deviation of displaced components in assembly.
Merely by way of example, the invention has been applied to placing
packaged devices onto electronic substrates for the manufacture of
electronic systems. But it would be recognized that the invention
has a much broader range of applicability.
[0003] Electronic devices have proliferated over the years. As the
complexity and the operation speed of integrated circuits (IC)
increase, it is not unusual to see an increasing number of devices
with pin-counts exceeding hundreds or even a thousand. For example,
a high-speed design requires more power and ground pins. The
differential pairs are replacing the single-end signals at the
input and output pins (I/O) of a device to meet the signal
integrity requirements. In addition, as system-on-a-chip becomes a
reality, more and more pins are added to the device I/O to supports
more functions. In generally, many if not all of these tend to
increase the number of pins in a packaged device or component.
[0004] As the device pin-count increases, the pin pitch of the
device tends to decrease to limit the outgrowth of package size.
The reduced pin-pitch poses a challenge for the placement equipment
to place components accurately on target platform, such as a
printed circuit board (PCB), especially if the pin pitch is smaller
than 0.5 mm.
[0005] The conventional surface-mount equipment uses the Cartesian
coordinates at the center of target land pattern as reference point
to place a component on PCB. There is no feedback to monitor the
accuracy of component placement. Without proper feedback, the
accuracy of component placement is uncertain. Actually, the
accuracy of component placement is influenced by the imperfectness
in package outline, the deviation of component's contact array from
ideal grid location, the imperfectness in PCB mounting references,
the aging and the intrinsic tolerance of placement equipment, and
so on. As the accumulative error is getting closer to the pitch
size of contact array, placing a component accurately on PCB is a
big challenge.
[0006] It is not uncommon to encounter component placement problem
in surface mount assembly line, especially for the placement of
fine-pitch components. For example, if a BGA component is
inaccurately placed on PCB, it could cause the BGA's contact array
to deviate from the idea land pattern location, resulting in either
inadequate soldering or solder bridging to adjacent pads on PCB. A
rework to fix these problems are tedious and expensive. It is even
worse for the rework of a pricy, high pin-count component on a high
density PCB.
[0007] Also, manufacturers frequently use sockets to house
high-end, high pin count chips on motherboard. It enables user to
choose proper speed grade component or to perform speed upgrade at
field. However, there is no handy method for users or manufacturers
to monitor if a chip has been properly inserted in the socket or if
the chip is in good contact with the receptacle inside the
socket.
[0008] It is seen that techniques for detecting and improving
component placement accuracy and for detecting the contact status
are desirable.
BRIEF SUMMARY OF THE INVENTION
[0009] The present invention relates in general to assembly
techniques. According to the present invention, the alignment and
probing techniques to improve the accuracy of component placement
in assembly are described. More particularly, the invention
includes methods and structures to detect and improve the component
placement accuracy on a target platform by incorporating alignment
marks on component and reference marks on target platform with
various probing techniques. A set of sensors grouped in an array to
form a multiple-sensor probe is used to detect the deviation of
displaced components on target platform. Merely by way of example,
the invention has been applied to placing components onto
electronic substrates for the manufacture of electronic systems.
But it would be recognized that the invention has a much broader
range of applicability, such the precision alignment of a set of
different objects.
[0010] In a specific embodiment, the present invention provides
solution for IC or packaged device with a set of one or more
alignment marks in predetermined spatial regions on the IC or
packaged device. Depending on the embodiments, the packaged device,
containing a plurality of I/O contacts, can be an integrated
circuit device encapsulated in a suitable material, such as plastic
(e.g., epoxy), ceramic (e.g., aluminum dioxide) or other materials,
and a plurality of bonding pads connected to its I/O contacts. The
packaged device can be a multiple-IC stacked device,
multiple-package stacked device, or multiple-chip carrier. The
packaged device can also be an integrated circuit device laminated
with an anisotropic conductive elastomer (ACE) membrane as
interconnect interface for external connections, and so on. The set
of alignment marks monitor the placement of such packaged device on
target substrate or platform to determine if the plurality of
external contacts on device are accurately placed at land pattern
on target substrate or platform, such as printed circuit board,
mother board, ceramic board, bare die IC, or other packaged device
or component. If the packaged device is inaccurately placed, the
alignment mark can also be used to obtain feedback to adjust the
device position deviation. For simplicity, the ICs and packaged
devices are called as components.
[0011] In a specific embodiment, an alignment mark is a reference
area on component that an external probe can use it to monitor the
accuracy of component placement on a target platform. The alignment
mark incorporated on component can be a conduction path connecting
the surface areas from top to bottom on component as a direct
alignment mark, or it can be a different conduction path connecting
two surface areas at the bottom of component as an indirect
alignment mark. Besides being a conduction path within the
component, the alignment mark can also be a simple surface marking
on component, depending upon the probing method used. The structure
of an alignment mark can be a simple geometric structure or a set
of geometric structures.
[0012] For each alignment mark on component, a reference mark can
be added to target platform for the component placement to refer
to. In a specific embodiment, the present invention provides
solution for target substrate or platform, such as printed-circuit
board, mother board, ceramic board, bare die IC, or other packaged
device or component, by incorporating a set of one or more
reference marks at pre-determined regions for the component
placement to refer to. The reference mark can be connected to other
reference marks on same target platform with a conduction path. It
can be connected to a ground point in a target platform, or it can
simply be a surface marking on the target platform, depending upon
the probing method used.
[0013] In preferred embodiments, the present invention provides
probe solutions for improving the accuracy in component placement.
The probe can be a single-sensor probe or can be an array of
sensors. A single-sensor probe can detect the accuracy of component
placement and the status of component contact condition on target
platform. A probe composing of an array of sensors to become a
multiple-sensor probe can adjust the deviation of displaced
component by aligning the one or more alignment marks on component
to the probe position that has been aligned to a reference mark. A
multiple sensor probe can detect the deviation of component and
feedback the information to the placement equipment to fix the
position deviation. While a single alignment mark can fix the
component displacement error, two alignment marks can fix the
placement orientation error. The probe can be a resistance probe, a
capacitance probe, an optical probe, or a combination of them. The
resistance probe is for on/off measurement, the capacitance probe
is for measuring the relative area overlapping between sensor
surface and target reference, and the optical probe is for
measuring the reflection from a target reference. The resistance
probe is a contact probe. The capacitance probe and optical probe
is a non-contact probe. The range of active sensors on the
multiple-sensor probe can be determined automatically by sensing
the size of target reference mark.
[0014] In a specific embodiment, the present invention provides
method for aligning component on target platform under various
probing techniques.
[0015] The method includes placing a component containing one or
more alignment marks to a region on target platform. The region on
target platform contains a target structure and a set of reference
marks to be monitored by probe to determine whether or not the
component has accurately been placed on the target structure. On
PCB, the target structure is a component land pattern. For each
alignment mark on component, a corresponding reference mark can be
included on target land pattern. The spatial relationship of the
alignment marking at the top to the contact array at the bottom of
component should match the spatial relationship of reference
marking with respect to contact array on land pattern. The method
also includes the way to align the multiple-sensor probe on target
land pattern, to determine the active range of the set of multiple
sensors, and to adjust the component position with respect to an
aligned probe.
[0016] Since the spatial relationship of alignment marks to the
associated contact array can be accurately controlled in the device
fabrication or component packaging process, a fixed spatial
relationship between the alignment mark and the contact array on
device or package can be ensured. This allows a package with minor
obliquity in physical outline (an oblique package) or a bare die
not perfectly centered-cut along the scribing line between adjacent
dies (off-center-cut die) can still be used for assembly. This is
because in assembly the component placement can then rely on the
position of alignment marks, rather than counting on the
geometrical tolerance of the package outline. In a specific
embodiment, the present invention provides a method for mounting a
physically out of specification device or package on a prospective
contact region on PCB or target platform. The physically out of
specification devices used to causing misalignment problem in
assembly could no longer be rejected. A larger tolerance in the die
cutting of device or in the molding of component package means a
higher component yield.
[0017] Many benefits can be achieved by way of the present
invention over conventional techniques. The present technique
provides an easy to use process that relies upon the conventional
technology. In some embodiments, the method provides a mean to
improve yields, reduce rework, and enhance placement accuracy.
Additionally, the method provides a process that is compatible with
the conventional process technology without substantial
modifications to the conventional equipment and processes. The
present invention is especially useful for the devices with
ultra-fine contact pitch and contact count exceeding hundreds or
even a thousand.
[0018] Moreover, the invention provides a method for monitoring the
placement of devices packaged with anisotropic conductive elastomer
(ACE) as interconnect interface. It enables the assembly of
non-solder-ball based packaged devices onto target platform or PCB
effectively and precisely and thereon for a variety of
applications. The anisotropic conductive elastomer contains a sea
of tiny conducting metal tubes embedded in elastic insulating
silicone membrane that conducts current only in certain direction.
It has been used as interconnect in high density and high pin count
test sockets to offer excellent contact, repeatability, and high
frequency characteristics in IC device test. It is feasible to be
as the interface interconnect for devices and packages. A device or
component laminated with ACE can be directly mounted onto a target
substrate. A clamp shell can be used to hold the devices together
without the need of soldering the devices on target substrate if an
accurate placement technology is achievable. The alignment
technique can make the assembly of ACE laminated devices on
electronics system feasible.
[0019] Depending upon the embodiment, one or more of these benefits
can be achieved and will be described in more detail throughout the
present specification. Various additional objects, features and
advantages of the present invention can be more fully appreciated
with reference to the detailed description and accompanying
drawings that follow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1A is an example of ideal package.
[0021] FIG. 1B is an example of oblique package.
[0022] FIG. 1C is a different example of out-of-specification
package with contact array off-grid.
[0023] FIG. 2 shows the placement of an off-grid component on land
pattern. All contacts on component are shifted by same offset from
land pattern after placement.
[0024] FIG. 3 illustrates the placement of component with a set of
conduction paths on a target platform with matching land pattern
and reference marks for checking the placement accuracy and contact
condition.
[0025] FIG. 4 shows a different configuration of alignment mark,
where all test accessible points are at the same side as the
contact array. It is simplified diagram of indirect alignment path
and indirect alignment mark.
[0026] FIG. 5 shows an example of using a single-detection-point
probe to check for the placement accuracy and the component contact
condition.
[0027] FIG. 6 shows an example of using a 4-point probe for
detecting the position deviation in component placement.
[0028] FIG. 7 shows a generalized test probe consisting of an array
of test points for detecting the deviation in component placement.
The active test points on the probe can be determined
on-the-fly.
[0029] FIG. 8 shows an example on the use of a set of SR latches to
track the positions of active test points and to align the center
of test probe to the center of target reference mark. The set of SR
latches and can be replaced by a set of latches with write
enable.
[0030] FIG. 9 illustrates the use of two sets of latches to
determine the direction and displacement of a deviated component.
One set of latches with status being shown in the first digit of a
binary pair indicates the overlap status between alignment mark and
reference mark. The other set of latches with status being shown in
the second digit of a binary pair indicates the position of active
test points.
[0031] FIG. 10 illustrates the use of three sets of latches to
align a small alignment mark to the center of a large reference
mark. The third digit in the tuple indicates the effective range of
active test points for a smaller alignment mark to align to.
[0032] FIG. 11 shows a graphical interpretation on the use of a
generalized probe to fix a deviated component, where the size of
alignment mark on component is smaller than that of reference
mark.
[0033] FIG. 12 is a simplified diagram showing a test probe
laminated with ACE material as interconnect interface to improve
the contact condition between the test points and the alignment
mark or between the test points and the reference mark.
[0034] FIG. 13 shows that incorporating two alignment marks on a
component can fix the orientation error in component placement.
[0035] FIG. 14 shows a simplified diagram of serial alignment chain
on target platform.
[0036] FIG. 15 is a simplified diagram of capacitance sensor, which
uses a guard to focus the sensor's electrical field.
[0037] FIG. 16 is a simplified diagram showing a set of capacitance
sensors grouped in an array to form a multiple-sensor capacitance
probe for use in non-contact electrical aligning.
[0038] FIG. 17 in an example on the use of electrical alignment
technique to monitor the component stacking
[0039] FIG. 18 shows a different application of electrical
alignment technique for aligning an object with quad-sensor
alignment mark onto the second object with a quad-triangle
reference mark.
[0040] FIG. 19 shows a transparent optical path on component can be
used as an optical alignment mark. If coupled with a matching a
reflective pad on target platform, it can be used to improve the
component placement accuracy.
[0041] FIG. 20 is a simplified diagram showing an array of
photo-detectors grouped in an array to form a multiple-detector
optical probe for use in component alignment.
[0042] FIG. 21 is a simplified diagram of optical setup with four
photo-detectors and four I/V amplifiers for the use in adjusting
the deviation of displaced components.
DETAILED DESCRIPTION OF THE INVENTION
[0043] According to the present invention, the alignment and
probing techniques to improve the component placement accuracy in
assembly are described. More particularly, the invention includes
methods and structures to detect and improve the component
placement accuracy on a target platform by incorporating alignment
marks on component and reference marks on target platform with
various probing techniques. A set of sensors grouped in an array to
form a multiple-sensor probe can detect the deviation of displaced
components in assembly. Merely by way of example, the invention has
been applied to placing packaged semiconductor devices onto
electronic substrates for the manufacture of electronic systems.
But it would be recognized that the invention has a much broader
range of applicability. Further details of the present invention
can be found throughout the present specification and more
particularly below.
Alignment Mark
[0044] According to preferred embodiments, an alignment mark is a
reference region on component that an external probe can use it to
monitor the accuracy of component placement on a target platform.
The alignment mark incorporated on component can be a conduction
path connecting the surface areas from top to bottom on component
as a direct alignment mark, or it can be a different conduction
path connecting two surface areas at the bottom of component as an
indirect alignment mark. Although the top-down conduction path does
not need to be a straight path, a straight path from top to bottom
or a matching top and bottom location on component is easier to
visualize the position of bottom contact point associated with the
alignment mark. The surface area of the conduction path at bottom
can be either the contact point on the contact array or a different
access point. Besides being a conduction path within the component,
the alignment mark can also be a simple surface marking on
component, depending upon the probing method used. The structure of
an alignment mark can be a simple geometric structure or a set of
geometric structures.
[0045] The alignment mark on component can be a single alignment
mark or a set of alignment marks. Normally, one alignment mark is
adequate. Second alignment mark is added to the component if the
component size is large or may require orientation adjustment to
improve the placement accuracy. The tangential deviation from the
ideal land pattern for far-end contacts in a large component could
be significant if there is an orientation error. For small
component, the impact due to a minor orientation error is
insignificant.
[0046] Depending upon the probing techniques, a probe can be either
in direct contact or not in direct contact with the alignment mark
when it is used to monitor the component placement accuracy. If a
resistance probe is used to monitor the position of alignment mark,
it requires a direct contact. If a capacitance probe is used to
monitor the position of alignment mark, it does not require a
direct contact. In both cases, the alignment mark is an electrical
alignment mark because there is a conduction current flowing
through the alignment mark, except one is a DC current and the
other is an AC current. If an optical probe is used to monitor the
reflection from the surface of alignment mark, then the alignment
mark is an optical alignment mark. Thus, a reflective, conductive
marking at the top surface of component can be used as an
electrical alignment mark or as an optical alignment mark depending
upon how the alignment mark is constructed and which probing
technique is used.
Reference Mark
[0047] For each alignment mark on component, a corresponding
reference mark can be added to the target platform for component
placement to refer to. The spatial relationship of alignment mark
to the contact array on component should match the spatial
relationship of reference mark to the associated land pattern on
target platform. The reference mark can be part of the component
contact land pattern on target platform. Depending upon the probing
technique and the application requirements, the reference mark may
or may not need to make a direct contact with the bottom of
alignment mark. In a special case if the reference mark is simply a
surface marking on target platform and if the alignment marking is
also a simple surface marking, the then the top position of
alignment marking at component can be chosen in such way that it
matches the center point or one of the corner points of contact
array on land pattern to eliminate the need of additional reference
mark on target platform.
Uncertainty in Component Placement
[0048] One useful aspect of current alignment techniques is that
the placement accuracy no longer relies on the component physical
outline, nor does it rely on the grid accuracy of contact array
from the edges of component. For example, if the molding process
causes a minor obliquity in the finished package such that the
contact array on package is not perfectly in parallel with the edge
of the package (an oblique package), it is difficult to place the
package correctly on land pattern under conventional technology,
especially when the package size is large and the array pitch size
is small; nor for a package with its contact-array shifted from the
ideal grid location (an off-grid package). Conventional placement
equipment uses the Cartesian coordinates at the center of land
pattern as a reference point to place component on it, based on the
assumption that the component physical outline is perfect and its
contact array is precisely located from all edges of component per
package mechanical specifications.
[0049] FIG. 1A shows an example of ideal package. It has a perfect
physical outline and all package contacts are on prescribed grid
locations. FIG. 1B shows an example of oblique package, where the
contact array is not in parallel with the package physical outline
and is tilted from the ideal grid location as viewed from the
package outline. FIG. 1C shows a different example of out of
specification package where the contact array is not properly
center-molded and thus bears an offset from ideal grid location as
viewed from the package outline. The packages in FIG. 1B and FIG.
1C are out of physical specification and is difficult to use in
conventional surface mount assembly. The imperfections in drawings
are exaggerated to illustrate the concept.
[0050] FIG. 2 shows an example of placing an off-grid component 200
on target land pattern 208. As shown, the contact array 209 on
component 200 suffers an offset 207 in the lower-left direction.
All contacts on component 200 will have the same offset 207 from
the target land pattern 208 after placement. This is different from
the placement of an oblique package where the skew or offset at the
corner contacts is larger than that at center.
[0051] Adding alignment marks to component could significantly
reduce the adverse impacts due to the imperfection in the package
outline and eliminate the contact position uncertainty caused by
the deviation of contact array from its ideal grid location even if
the package outline is still in spec. The alignment mark can be
placed within or beyond the component's contact array. It is
fabricated at a predetermined spatial relationship with respect to
component's contact array, rather than at a location based on the
component physical outline or the distance from the edge of
package.
[0052] The alignment mark decouples the equipment's reliance on the
ideal physical outline of components. Thus, some physically out of
specification components that were scraped in production before,
such as packages with minor obliquity in physical outline, packages
with contact array deviated from ideal grid location, or bare dies
not perfectly centered cut along the scribing line between adjacent
dies, could become useable for system assembly. Further detail of
the structure and the operation of alignment mark are described as
follows. Certain methods and variations are also provided
throughout the present specification.
Component Alignment Using Resistance Probe
[0053] In this embodiment, an alignment mark is an electrical
conduction path incorporated in component or package at a
pre-determined spatial location with respect to its contact array.
The conduction path can be constructed from the top surface of
component, where a test signal can be applied to, to the bottom
surface of component, where a matching contact or reference pad on
target platform can be connected underneath. The conduction path on
component and the matching reference pad on target platform are
paired to monitor the accuracy of component placement. If a
component were accurately placed on target platform, the matching
reference pad would appear underneath the bottom surface of the
conduction path and a conduction current would be detected when a
voltage were applied to the top surface of the component.
[0054] FIG. 3 shows an example of placing a component 300 on target
platform 310. The component 300 is an encapsulated package 301 with
a plurality of external connections at contact array 309 and one or
more conduction paths 302, 303 as alignment marks. On the target
platform, a land pattern 308 and a set of reference pads 306, 307
are pre-fabricated. The reference pads 306, 307 would be underneath
the bottom surface of conduction paths 302, 303 if the component
300 were aligned to land pattern 308. This diagram is merely an
example, which should not unduly limit the scope of the claims
herein. One of ordinary skill in the art would recognize many
variations, modifications, and alternatives, such as the component
can be a bare die with an embedded conduction path from top to
bottom as an alignment mark, an integrated circuit encapsulated in
plastic package or in ceramic material package with solder bumps,
or a device laminated with a layer of ACE membrane for external
interconnect but without solder bumps, or any combination of these,
and the like.
[0055] The conduction path in component 300 can be a straight pipe
or any irregular shape of conduction trace 303 running from top to
bottom in component. One or more conduction paths can be
incorporated in component. The external accessible points 304, 305
associated with the conduction paths 302, 303 can be a simple
circular pad. Other pad shape, such as square, rectangular,
triangle, trapezoid, or a combination of these, is likely.
Resistance probe is placed on the external access points 304, 305
to monitor the status of placement accuracy. The conduction path
can be a metal, a doped semiconductor path, or other detectable
entity.
[0056] In FIG. 3, a set of reference pads 306, 307 is
pre-fabricated on the target platform 310 as target reference to
guide the component placement. The reference pad can be a simple
circular pad, although other shape, such as square, rectangular,
triangle, trapezoid, a set of ground dots, a combination of these,
is also likely. The reference pads 306, 307 on target platform 310
are named as the "reference marks". For an alignment mark on
component, a correspondent reference mark can be added to the
target platform. The location of reference mark on target platform
310 is pre-fabricated in such a way that the spatial relationship
between reference marks 306, 307 and land pattern 308 on target
platform matches the spatial relationship between the bottom
contact of alignment marks 302, 303 and the contact array 309 on
component 300.
[0057] The size of reference mark is determined by the variance of
the placement system, such as the imperfections in component
outline, contact array, target platform, equipment precision,
mechanical aging, and database rounding error, etc. A system with
larger cumulative placement uncertainty requires a larger reference
mark. The size of reference mark should be large enough that at
each component initial placement the alignment mark should be at
least partially within the boundary of target reference mark. The
initial component placement is the placement based on the
coordinates of reference mark or target land pattern stored in the
placement database. In addition, the size of alignment mark is
preferably same as or smaller than the size of reference mark,
since it can impact the accuracy of component placement to be
explained later.
[0058] Besides a top-down structure, where one test point
associated with the conduction path is at the top surface of
component, it is also possible to have both test points 411, 412 at
the same side as the contact array 409, as shown in FIG. 4. Two
additional traces 406, 407 are added to the target platform 410 in
this case to connect the test points 411,412 associated with the
conduction path to the external accessible points 403, 404 on
target platform 410. The power and ground tips of a resistance
probe can be applied to the two external accessible points 403, 404
to detect if there is a conduction current to determine if the
component has been correctly placed on target land pattern 408. The
conduction path with both test points located at the same side as
contact array is named as an indirect alignment path. Its contact
points are the indirect alignment marks.
[0059] If several indirect alignment paths on different components
are connected into a serial daisy chain on target platform, it can
be used to monitor the contact condition of all components in the
chain or to check if all components are still in place when the
system is in use. This is useful for electronic system assembled
with solderless components, such as components using ACE as
interconnect interface, where the indirect alignment paths for all
ACE based components can be connected in a single alignment chain
or several shorter chains. A supply voltage and detectors, such as
LED diodes, can be connected to each chain to monitor the
connection status of all solderless components in the chain.
[0060] Incorporating alignment marks on component not only can
detect the accuracy of component placement on target platform, but
also can fix the component position deviation after placement.
However, it is the configuration of test probe and the handling of
signals detected by test probe to determine the functions of
alignment mark. While a single-test-point probe, namely a
single-sensor probe, can detect the placement status and the
interface contact condition, a multiple-test-point probe, namely a
multiple-sensor probe, can detect the direction and the magnitude
of deviation in component placement to be explained later.
[0061] FIG. 5 shows an example of using a single-test-point
resistance probe 520, to monitor the alignment status of component
500 on target platform 510. Assume a component 500 containing two
alignment marks 501 and 502 has aligned to a land pattern on target
platform 510, where there is a set of corresponding reference marks
503 and 504. A conduction path is formed from the supply tip 521 of
probe 520, through alignment mark 501, to reference mark 503 on
target platform 510. A second conduction path is from alignment
mark 502 to the reference mark 504 on target platform 510.
[0062] To avoid the two reference marks 503, 504 from floating at
test, both should be connected to an internal plane 507 on target
platform 510, which is connected to a known reference voltage or
ground 508 through a surface contact 505. A different approach is
to connect the two reference marks together internally. A probe can
be applied to test the connectivity after a component is placed.
For example, if the supply tip 521 of probe 520 is applied to the
top surface of alignment mark 501 and its ground tip 515 is applied
to the top surface of alignment mark 502, then a closed current
loop is formed to detect a conduction current from the probe's test
tip 521, through alignment 501, reference mark 503, internal trace
or plane 507, back to reference mark 504, alignment mark 502, and
finally to the probe ground tip 515, if the component 500 is well
aligned on target platform 510. Although two alignment marks are
shown, one alignment mark is adequate if the orientation error is
less likely in the component placement.
[0063] Alternatively, if the reference pads 503, 504 are connected
to an internal plane 507 and a reference voltage or ground 508 is
applied to the surface contact 505, then the reference pads 503,
504 will be at the reference voltage or ground. As the test tip 521
is applied to the top surface of any alignment mark on component
500 in this case, a closed loop is formed from the probe, through
alignment mark, corresponding reference mark, to system ground and
then probe ground, provided that the two grounds are connected
together.
[0064] As shown in FIG. 5, a test probe 520 may contain a resistor
522, a light-emitting diode 523, a sound buzzer, or a current meter
524 to indicate the status of placement. The resistor limits the
maximum current to flow through the probe and the alignment mark.
The light-emitting diode provides a visible signal to indicate if
it is an accurate placement. The sound buzzer enables users to
hear. The current meter shows the amount of current through the
probe. The probe can be built in the placement equipment for
monitoring the placement status. It can also be a probe for
checking the component placement accuracy and contact condition by
users manually.
[0065] FIG. 6 illustrates the use of a 4-point probe to improve the
accuracy of component placement. A multiple-detection point probe
is essential to fix the component deviation in placement. A 4-point
resistance-probe with test points A, B, C, and D is used in this
example. A circular reference mark 602 is shown in large dot
circle. An alignment mark 601 also in circular shape is shown in
large solid circle. To make the illustration simpler, assume that
the size of electrical alignment mark 601 is the same as the size
of reference mark 602, and also assume that the probe used in this
placement has all four test points match the circumference of
reference mark 602. If a square reference mark is chosen as an
example, then all four test points on probe shall be able to match
the four corners of square reference mark after the probe position
is aligned with the reference mark.
[0066] To fix the deviation in component placement, first it needs
to align the probe position with respect to that of reference mark
by moving all four test-points A,B,C,D on probe onto or adjacent to
the boundary of target reference mark 602. Then, a component is
moved in and placed at the coordinate of target land pattern per
the information stored in placement database. If there were a
deviation in component placement, the alignment mark 601 on
component would only partially overlap with the target reference
mark 602 on target platform. The 4-point resistance probe can
detect this partial overlapping. For example, assume the deviation
is toward the lower, left corner and only the test point C at the
lower, left corner is in contact with the alignment mark 601. Then,
only the test point C detects a current flow through the alignment
mark 601 to ground reference mark 602. There is no current being
detected by the test points A, B, and D. This is because they are
in a region outside the alignment mark 601, which is not
electrically conductive. Using this test result as a feedback, the
placement equipment can thus know the deviation of component
placement. The shifting of component position upward and right-ward
can fix the placement deviation, i.e., from the direction current
being detected through alignment mark 601 by test points to the
direction no current being detected. This process continues until
the alignment mark 601 on component is moved to a new position in
contact with all test points. Then, the component is accurately
positioned.
[0067] If after the placement the alignment mark 601 is at a
location that two test points detect conduction current, say the
two left test points. Then the alignment mark, so is the component,
should be shifted rightward until it reaches a position that all
four test-points can detect the current flow. In this case, the
test probe, the reference mark and the alignment mark are all
aligned and the component is accurately placed. The test probe and
the component's pickup header in placement equipment can both be
adjusted independently.
[0068] Adjusting test probe in line with target reference mark at
the beginning of each component placement is essential to ensure
the placement accuracy. In this special case it can be done,
without a component between probe and reference mark, by making the
probe in contact with the reference mark, the deviation from the
center of probe to the center of target reference mark can be
determined by monitoring which test points conduct current. By
shifting the probe position toward the direction that no current
being detected by test points, the probe can be aligned with the
target reference mark. This is a probe alignment step.
[0069] A multiple test point probe has more capability than a
single test point probe in monitoring the component placement.
While a single-test-point resistance probe can be used to detect
the placement accuracy and the contact condition, a
multiple-test-point probe can be used to detect and provide
feedback on the deviation of a displaced component.
[0070] FIG. 7 shows a generalized test probe consisting of an array
of test-points 701. If the area detectable by the array of test
points 701 is larger than the size of reference mark 700, then only
a subset of test points in the array will be activated to detect
the position deviation. The subset of test points to be activated
in each component placement can be determined automatically
on-the-fly in a similar manner. This can be done by lowering the
test probe--assume it is a resistance probe--in contact with
reference mark 700 and applying a voltage to all test points in
array 701 to see which test point conducts current.
[0071] If all test points conducting current were the inner test
points in the array, then the inner test points detecting current
702 could be chosen to facilitate the component position
adjustment. Those test points in the array not conducting current
flow 703 could be ignored in the current component placement. The
subset of inner test points in array being activated to facilitate
the component position adjustment is the active test points 702.
Those test points being ignored in the position adjustment are the
inactive test points 703.
[0072] Since the center of a multiple-test-point probe is known
from the probe structure, the probe design can be simpler if the
set of active test points could be repositioned to center around
the center of multiple-test-point probe. This can be done by moving
the probe to a new position until all inner active test points are
in the central region of a multiple-test-point probe. In this new
position the center of test probe is aligned with the center of
target reference mark and with the outer range of active test
points matching the boundary of the reference mark.
[0073] If part of test points conducting current were at the edge
of multiple-test-point array, then either the size of probe is too
small or the probe position is not lined up with the reference
mark. Assume the size of probe is too small, i.e., smaller than the
size of target reference mark, it could cause an uncertainty in the
component position adjustment due to unable to align a small probe
to the center of reference mark. Preferably, the range of test
probe is compatible with or is larger than the size of reference
mark. And if the probe position is not lined up with the reference
mark, then it should be adjusted until the center of active test
points in array is lined up with the center of the reference
mark.
[0074] A set of asynchronous set-reset (SR) latches can be
implemented in the electronics of test probe, one SR latch for each
test point on probe. Corresponding to the subset of test points on
probe being activated to monitor the component placement, a subset
of SR latches is set to indicate the positions of active test
points for each component placement. The entire set of SR latches
is cleared at the start of each component placement. A latch with
write-enable can also be used to implement the function of SR
latch.
[0075] FIG. 8 illustrates a method of using the set of SR latches
for aligning a test probe to the center of target reference mark.
To align a multiple-test-point resistance probe to a reference
mark, after making a contact with reference mark, the SR latch is
set if the corresponding test-point conducts current and the SR
latch remains reset if the corresponding test-point detects no
current. Counters can be used to scan all rows and columns on the
set of SR latches to find out which row and which column detect the
highest number of ones. The interception of the highest row and
column is the center of reference mark. A row counter, a column
counter, a comparator, registers for tracking the row number and
the column number that have the highest number of ones, and a set
of multiplexers to select proper inputs to these logic, etc, can be
used to achieve such function. This is merely an example, which
should not unduly limit the scope of the invention. By comparing
the row number and the column number that have the highest number
of ones with the row number and the column number at the center of
probe, the displacement to align the probe to the center of
reference mark can be determined. The probe can thus be shifted
accordingly to align with the center of reference mark. After the
re-positioning of probe, the SR latches are cleared to record the
new conduction status for all test points on probe. The SR latches
being set at this stage contain the information of the set of
active test points aligned with the center of reference mark.
[0076] For aligning component, another set of transparent latches
can be added to the probe electronics to track the instantaneous
conduction status of all test points each time the component
changes to a new position. The contents of transparent latches
record the overlap status between the alignment mark and the
reference mark. Their contents will be compared with the contents
of SR latches. If the contents of both sets match, then the
component is accurately aligned on the target land pattern.
[0077] FIG. 9 shows an example of using two sets of latches to
derive the direction and displacement of a deviated component. The
first digit in the binary pair is the status of the set of
transparent latches, which indicates the overlap status between the
alignment mark and the reference mark. The second digit in the
binary pair is the status of the set of SR latches, which indicates
the position of active test points.
[0078] By comparing the contents of these two latches, or the value
of the binary pair, the direction and the adjustment displacement
for a misplaced component can be derived. To make the explanation
simpler, let's assume the size of alignment mark is the same as the
size of reference mark. A "1" in the first digit indicates the area
that the alignment mark overlaps with the reference mark. A "1" in
the second digit indicates the location of active test points. By
referring to the "11" region in FIG. 9, which is known by a simple
AND logic, there are three columns to the right side containing all
"01"s and only two rows to the up side containing all "01"s inside
the reference mark range. The placement equipment thus knows that a
shifting of three positions to the right side and two positions to
the up side can re-position the component correctly. The position
of test points containing "01" can be known by a simple exclusive
OR gate. This illustration is merely an example, which should not
unduly limit the scope of the invention.
[0079] While a larger reference mark can extend the range of
adjustment for a deviated component, a smaller alignment mark can
still work properly if test probe is large enough, i.e., if the
range of all test points on probe is larger than the size of
reference mark. To support a smaller alignment mark, a third set of
latches can be incorporated in the probe electronics to indicate
the effective sub-range of active test points where the small
alignment mark will be referred to in alignment. The third set of
latches servers as an enable mask in determining the effective
range of "01"s in the derivation of the component shift
displacement.
[0080] In FIG. 10, the third digit in the binary tuple indicates
the sub-range inside reference mark that matches the size of
alignment mark. A "1" in the third digit, such as the binary tupes
"111" and "011", indicates the location of the sub-set of active
test points for use as the target for a smaller alignment mark to
align to. While the entire range of active test points, determined
by the size of reference mark, is used to detect the presence of
alignment mark, it is the sub-range of active test points being
used as the target for the small alignment mark to align to. A "11"
in the first two digits of tuple, i.e., "11x", as shown in the
example of FIG. 10, represents the overlapping area between the
alignment mark and the reference mark. The tuple "011" indicates
the area that the alignment mark is still not in line with the
reference mark. There are two columns to the right side of "111"
containing "011"s and two rows to the top side of "111" containing
"011"s in the target sub-range in FIG. 10. Thus, a shifting of
component two positions upward and two positions rightward can
still fully align the alignment mark to the reference mark, even if
the alignment mark is smaller than the reference mark.
[0081] The actual sub-range of active test points associated with
the smaller alignment mark can be derived by referring to the
diameter information of alignment mark stored in the placement
database or by using an image sensor associated with the placement
equipment to measure the diameter of the alignment mark
on-the-fly.
[0082] FIG. 11 shows a graphical interpretation on the use of a
generalized test probe to fix the position of a deviated component.
The alignment mark 1101, which reflects the position of component,
is smaller than the reference mark 1102 in this example. Before
each component placement starts, the range of active test points on
probe should be determined in advance, i.e., by moving the probe on
top of the reference mark 1102 but without component in between and
applying a voltage to all test points to see which one conducts
current. Since the center of test probe is known, the set of active
test points can be chosen to have the probe in line with the center
of reference mark 1102 and to have the outmost active test points
adjacent the boundary of reference mark 1102. In FIG. 11, the
inactive test points on probe are shown as dot circles and the
active test points are shown as solid circles and solid black dot.
The sub-range of active test points to be used for aligning a
smaller alignment mark to the reference mark can also be derived
on-the-fly by using an image sensor inside the placement equipment
to measure the diameter of the alignment mark or by referring to
the diameter information of alignment mark stored in the placement
database.
[0083] After initial component placement, the alignment mark 1101
on component is either entirely or partially within the range of
active test points being defined by the reference mark 1102. Assume
there is a deviation in the initial component placement and assume
only one corner active test point 1105, shown in the solid black
dot in FIG. 11, detects current flow. The placement equipment could
then use the subset of the active test points as target to shift
the alignment mark rightward and upward until the alignment mark
matches the entire subset of active test points 1106, shown as the
big, central dot circle in FIG. 11. Thus, the component can still
be accurately aligned to a target land pattern even if the
alignment mark is smaller than the reference mark.
[0084] On the contrary, if the area of alignment mark were much
larger than the area of reference mark, the placement accuracy
would be degraded. This is because the range of active test points
defined by a small reference mark is rather limited. It is unable
to proceed the component adjustment process because no adequate
feedback is available to instruct the large alignment mark on the
component to move its center to line up with a limited set of
active test points. For resistance probing, the effect of using a
larger alignment mark is similar to the effect of using a
single-test-point probe, where the area covered by a single test
point probe is very narrow, similar to the effect of a smaller test
probe or a smaller reference mark. Nevertheless, a smaller
reference mark can detect much higher placement accuracy under a
single-test-point probe, unless the size of reference mark is too
small to be detectable.
[0085] In case after initial placement the component is placed
beyond the reference mark. No conduction current is detected by any
of the active test points in this case. This indicates either the
placement equipment is improperly set up or the worst-case
deviation in the component placement is under-estimated to cause
the alignment mark step beyond the under-sized reference mark. An
improper system setup or operational error can thus be detected
easily.
[0086] To improve the durability of test probe and to ensure good
contact to alignment mark and reference mark, an anisotropic
conductive elastomer (ACE) 1205 can be laminated on the surface of
a generalized resistance probe 1200 as the interconnect interface,
shown in FIG. 12. In this figure, each test point consists of a
conduction pipe 1202 inside the probe 1200. This figure is merely
an example, which should not unduly limit the scope of the
invention. For example, the probe is shown in circular shape, but
other probe shape, such as square, rectangular, triangle,
trapezoid, other irregular shape, or even a flexible cable, is
likely. Also, although the test points are shown in a regular array
in the figure, the test points in array can be arranged in any
configuration based on the requirements of applications.
[0087] To resolve the orientation error in component placement, two
alignment marks can be incorporated at the opposite sides of
component. It is more likely to encounter an orientation error for
the components with a large contact array or ultra-fine contact
pitch.
[0088] FIG. 13 shows an example of incorporating two alignment
marks on component to fix the orientation error in component
placement. Assume that the component is tilted counter-clockwise
after initial placement and assume that the size of alignment mark
is the same as the size of reference mark. Two large solid circles
in the diagram represent two alignment marks 1301, 1302 on
component. An artificial solid line linking these two-alignment
marks 1301, 1302 is drawn to show an imaginary orientation of the
component after initial placement. Two corresponding reference
marks 1303, 1304 on target platform are shown in large dot circles,
connecting by an artificial dot line to indicate the ideal target
orientation. The acute angle intercepted by the solid line and the
dot line is the orientation error after initial placement. Assume
that there is no displacement error, then the interception point is
the center of the component 1300. If assume two test probes are
used to monitor the placement of a component containing two
alignment marks, although one is adequate if the active test points
and the center of test probe can be recorded by the placement
equipment for each probing.
[0089] Before placing a component, the probes must be aligned with
the target reference marks. For resistance probe, this is done by
making the probe in contact with reference mark to determine the
range and the coordinate of active test points with respect to the
center of reference mark. Then, a component is picked and placed at
the target land pattern with the probe moved up. If there is no
orientation error in the component placement, both alignment marks
1301, 1302 will stay exactly on top of reference marks 1303, 1304
and match the range of active test points. If there is an
orientation error, then some of the active test points on probe
will be outside the range of alignment mark and conduct no current.
By monitoring the conduction status of active test points, the
direction of adjustment can be known, i.e., by rotating component
or alignment mark toward the side that active test points detect no
current, provided that the component pickup header in assembly is
capable of performing angular position adjustment.
[0090] In FIG. 13, the two alignment marks 1301, 1302 on component
are in the upper right and lower left locations. If a
counter-clockwise orientation error occurs after component
placement, then the active test points 1305 detecting conduction
current will lean at the left-hand side for the upper probing and
the active test points 1307 detecting conduction for the lower
probing will lean at the right-hand side. By rotating the component
from the side that active test points detect current to the side
detecting no current, i.e., in the direction of clockwise rotation,
the component orientation error can be fixed. But were if all
current sensing tests points appear at the same side under both
probing, say all at the left-hand side, then a displacement error
had taken place and the component should be shifted right-toward to
fix its displacement error.
[0091] The alignment technique can be adaptive to application
needs. For example, if the placement system has intrinsic skew such
that the placement error is larger in one direction than the other,
then a rectangular reference mark can be chosen with its long side
corresponding to the direction of larger deviation. Preferably, the
size of probe is larger than the size of reference mark in order to
align the probe and to determine the range of active test points on
probe automatically. Also preferably, the size of reference mark is
larger than or is about the size of alignment mark.
[0092] The resistance probing that requires direct contact
measurement may cause contamination of solder paste around target
land pattern in case it needs to move the displaced component
around on target platform. There is no such problem for the
components laminated with ACE as interconnect interface. The
components laminated with ACE interconnect interface also makes a
good contact with the land pattern on target platform.
[0093] To avoid the adverse effect of potential solder paste
contaminations, it is also possible to laminate a thin layer of
solder material on the surface of land patterns on PCB, similar to
the solder ball material on package, during PCB fabrication. Then,
the solder paste-printing step can be eliminated in surface-mount
assembly, thus free from solder paste contaminations.
[0094] The top-down alignment mark and the indirect electrical
alignment paths can be combined and incorporated in the same
component. The top-down alignment mark can improve the placement
accuracy and the indirect alignment paths can be linked in a serial
alignment chain to monitor the contact status of all components in
the chain, as shown in FIG. 14. This is particular useful for the
electronic systems assembled with components laminated with ACE
interconnect.
Alignment Using Capacitance Probe
[0095] The electrical alignment can also be achieved by using
non-contact method. Rather than using a resistance probe to measure
the on/off resistance at the direct contact point, a capacitor
probe can be used to align component in non-contact approach.
[0096] FIG. 15 shows an example of capacitance sensor manufactured
by Lion Precision of St. Paul, Minn. To improve measurement
accuracy, the electrical field from the sensor needs to be confined
within the space between the sensor's surface and the reference
target. A separate conductor kept at the same voltage as the sensor
itself is used as a guard to surround the sides and the back of the
sensor. When an AC signal is applied to the sensor, a separate
circuit applies exact same excitation voltage to the guard. Because
there is no voltage difference between the sensor and the guard,
there is no electrical field between them. Any other conductors
beside or behind the capacitance probe form an electrical field
with the guard, instead of the sensor. Only the unguarded front end
of the sensor forms an electrical field to a reference target. The
sensor creates an electrical field that is a projection of its size
and shape. For instance, a round sensor could project a cylindrical
electrical field to a reference target. In Lion Precision's
capacitor sensor, the cylindrical electrical field could spread up
to 30% at an effective range of 40% of the sensor diameter.
[0097] As an AC signal is applied to the sensor surface, AC current
will flow through the capacitance formed in the gap between the
sensor and the target reference. The amount of current that flows
is dependent on the amount of capacitance between the sensor and
the target reference, i.e., determined by the size of separation
and the extent of overlap between the surfaces. If the senor is
fully aligned with the reference target, the capacitance formed in
the gap is the largest and the AC current will be the highest. If
the sensor is entirely beyond the reference target, there is no
surface area overlap and the capacitance is irrelevant, provided
that no foreign conductor besides the reference target is within
the sensor range.
[0098] A set of guarded capacitance sensors can be grouped in an
array to form a multiple-sensor capacitance probe, as shown in FIG.
16, for non-contact electrical probing. The sensors in the array
can also be selectively activated per applications requirement. For
example, FIG. 16 shows a nine-sensor probe. But if only the sensors
at the four corners of probe are activated for placement detection,
then it becomes a four-sensor capacitance probe.
[0099] For probing alignment, the exact capacitance detected by
each sensor on probe is not a major concern. This is because in
alignment applications the capacitance sensor is not to measure the
exact separation between two parallel objects, where a precise
capacitance reading is essential. It is the relative capacitances,
determined by the extent of area overlapping between the probe
surface and the target reference area, plays a key role. The
effective range of a multiple-sensor capacitance probe for
measuring the placement accuracy is greater than that of a single
sensor, provided that no foreign conductor is within the range of
sensors' electrical field to disturb the relative capacitance
measurement.
[0100] As an AC excitation is applied to all active sensors on
probe, all sensors would detect same capacitance between sensor and
target reference and thus would measure same amount of AC current
if the probe were fully aligned with the reference target. If
misalignment exists, the portion of active sensors on probe
detecting area overlap with the reference target would conduct the
highest current while the portion of active sensors beyond the
reference target would conduct a minimal current. By comparing the
relative magnitude of AC currents among the array of active
sensors, the direction of deviation can be known. The deviation can
be fixed by aligning the component alignment mark under the
capacitance probe or by moving the displaced component toward the
direction where the active sensors detect lower level of AC
current. It does not need to have the probe in contact with target
platform or component in this process.
[0101] The capacitance sensor is normally calibrated to a grounded
target. The body or outer jacket of capacitance probe should be
electrically wired to ground to improve the probe accuracy. The
reference target also requires properly grounding. A reference
target not properly grounded could degrade probe's sensitivity and
accuracy. Fortunately, many targets, while not directly grounded,
have a significant capacitance to ground though its environment,
such as the component pickup header. The large capacitance
eventually shorts the target reference to ground under AC
excitation. Thus, connecting the reference mark on target platform
to a ground plane or the alignment mark at component surface to its
local ground is helpful but not essential as long as the ratio
between the target's capacitance to ground and the target's
capacitance to sensor is reasonably large. A ratio over 10 will
have the error in capacitance measurement less than 10% as seen in
the case of two capacitances in series.
[0102] Using capacitance probe to align component, only the surface
marking at the top of the component is relevant. The surface area
of alignment mark on top of component determines the capacitance
value detected by a capacitance sensor. It no longer requires a
conduction path from top to bottom on component as an electrical
alignment mark, as seen in the case of resistance probing. However,
the Cartesian coordinate relationship between the alignment marking
at the top and the contact array at the bottom of component should
match the coordinate relationship between the reference mark and
the target land pattern on target platform. To align an alignment
marking under an capacitance probe can be done by moving the
alignment marking around under capacitance probe, assumed the probe
has already aligned with a reference mark, and monitoring where all
active sensors over the alignment marking detect same amount of
currents, which exceeds certain minimum threshold. After the
alignment marking is in line with probe, the aligned component
could then be lowed down to place on target land pattern
accurately.
[0103] Using a non-contact probing, such as capacitance probing,
aligning the probe to target reference and aligning the alignment
marking to probe can be done independently. Thus, the size
relationship between the alignment marking on component and the
reference mark on target platform is no longer as critical as the
case of contact probing, provide that the probe detection range is
larger enough to cover both of them. Thus, a component with a large
alignment mark can be still aligned accurately on a smaller
reference mark under a non-contact probing.
[0104] The electrical aligning technique can also be used to
monitor the accuracy in the stacking of packaged chips, where a
conduction path can be inserted from the top to bottom on the
packaged chip as an electrical alignment mark. The electrical
alignment technique can also be used to monitor the accuracy in the
bare-die stacking, where a conduction path from top to bottom in
bare-die can be created using ion implantation, diffusion, or other
method.
[0105] FIG. 17 shows an example of using electrical alignment
technique to monitor the stacking of components. Several variations
are shown in the example to illustrate the applications under
different situations. The accuracy of component stacking can be
known by directly placing a probe at the top of the top component
alignment mark and at the bottom of the bottom component alignment
mark. If the stacked components are to be placed on a supporting
plane, the alignment mark at the bottom of the bottom component can
be connected to a ground reference point on the supporting plane to
create a monitoring path from probe to ground. Or a pair of
electrical alignment marks can be incorporated on the components to
be stacked, which can be connected to a conduction path in the
supporting plane for probe to monitor the stacking accuracy from
the top of both alignment marks on stacked components. The little
circle in the figure indicates contact pints to other stacked
component or supporting plane.
[0106] FIG. 18 shows a different application of using the
electrical alignment method to align two objects together. In this
example an array of 4 sensors is incorporated in the first object
and an array of 4 triangles as reference target is incorporated in
the second object for monitoring the alignment accuracy. The four
reference triangles can be separated as four reference marks or can
be butt together to form a single one as shown. The four sensors
can be a set of resistance probe in contact measurement or can be a
set of capacitance probe in non-contact approach.
[0107] The setup works in a similar manner. Assume the first object
is deviated from the second object in the lower, left direction to
cause the two left alignment sensors on the first object outside
the quad-triangle reference mark associated with the second object.
If assume the range of sensor array and the range of reference
array are matched, then an instruction to shift the first object
toward the right-hand side direction could be generated by the
sensing electronics. But after the first object is shifted toward
the right-hand side, the two lower right sensors would then step
beyond the reference mark region (triangles C, D) on the second
object, but with the two upper sensors still within the range of
reference triangles at this moment. By moving the first object
upward, i.e., toward the direction that the sensors have detected
currents, the four alignment sensors can then be moved into the
range of reference mark and the two objects are finally
aligned.
Alignment Using Optical Probe
[0108] If the alignment marking on top of component and the
reference marking on target platform are highly reflective, an
optical probe can be used for the component alignment. The
alignment mark can also be a transparent optical path 1901 on
component that enables a laser beam to pass through and reflect
back from a reference mark underneath, as shown in FIG. 19. Either
a reflective surface marking on top of component or an optical path
on component can be used as an optical alignment mark.
[0109] In optical probing, it does not need to ground the alignment
mark or the optical path on component, nor is there a need to
ground the reference mark on target platform. However, the spatial
relationship between the alignment mark at top and the contact
array at the bottom of component should match the spatial
relationship between the reference mark and the associated land
pattern on target platform, same as other probing techniques.
[0110] FIG. 20 shows a set of photo-detectors or photo-diodes
grouped in an array to form a multiple-sensor optical probe for
non-contact probing. The sensors in the array can be selectively
grouped per the resolution requirements. For example, FIG. 20 shows
a sixteen-sensor probe. But if the boundary of the reflection beam
is out of focus and is slightly blurred or if the size of
reflection beam is rather large, the sixteen photo-detectors on the
optical probe could be rearranged into four groups to form a
four-senor optical probe. The re-arrangement of sensors can be done
by re-arranging the input signals to the probe electronics. In a
different example, if the incident beam can be adjusted and
focused, only the four sensors at the center of the probe could be
chosen as the target sensors of alignment to increase the alignment
accuracy, although the entire range of all 16 sensors are used as
active sensors to increase the incident beam search range.
[0111] If the generalized test probe, consisting of an array of
test sensors, is a resistance probe, it measures the connection
resistance or DC current at each contact point. If it is a
capacitance probe, it measures the AC current at each test sensor,
which reflects the area overlapping between the surface of each
sensor and the target reference. If it is an optical probe, it
detects the photocurrent at each photo-detector, which indicates
the position of the reflection beam.
[0112] Using optical alignment, the probe position must be aligned
with the center of reference mark on target platform at the
beginning of component placement. The reference mark can be a
simple reflective pad in optical alignment. Ideally, the diameter
of incident light beam shall be compatible with the size of
reference mark. Otherwise, an uncertainty takes place, regardless
the size of incident laser beam is too large or the size of
reference mark is too large. On the detector side, the incident
beam shall be properly focused to ensure it is within the range of
optical detectors. After aligning the probe to the center of
reference mark, a component is moved in and placed at the land
pattern. Next is to align the position of alignment mark with
respect to the position of the probe. For non-contact probing, it
is the two-step indirect alignment method that determines the
ultimate accuracy in component placement. Depending upon how the
light beam is collimated in the optical approach, the relative
sizes of alignment mark and reference mark are not as critical as
the case of resistance probing.
[0113] FIG. 21 shows an example of optical pickup containing a
four-photo-detector probe and associated I/V amplifiers for
detecting the deviation of displaced components. Each I/V amplifier
includes a current-voltage converter and a voltage amplifier. As
light incidents on the photo-detector on probe, a current is
generated, which is then amplified by the I/V amplifier. A circular
light beam 2002, reflected from a target reference is also shown.
The reflected beam will be at the center of optical pickup if the
component is fully aligned to the reference target.
[0114] Four different voltages, VA, VB, VC, and VD, are generated
by the optical pickup in FIG. 21. A set of comparators can be used
to compare the voltage output from these I/V amplifiers. The
direction of adjustment can be determined by comparing the output
voltage of each I/V amplifier with a predetermined threshold
voltage. A high output voltage detected by the comparator indicates
the probe and the target reference has area overlap. A low output
voltage detected by the comparator indicates the corresponding area
is still misaligned. By shifting the component from the position
that comparator detects high output voltage to the position that
comparator detects low output voltage, the misalignment can be
fixed. If none of the comparators detect a voltage level that
exceeds the pre-determined threshold level (VT), i.e., no relevant
reflection beam is detected by any of four photo-detectors, it
indicates the component is entirely misplaced or the optical probe
is not in line with the target reference at all. The component is
accurately aligned on the target platform if all four voltage
levels reach the pre-determined threshold level.
Alignment Procedure
[0115] The procedure of using alignment technique to improve the
accuracy in component placement is summarized as follows: [0116] a.
In preparation for component placement in assembly, some relevant
information, such as the coordinate of the center of land pattern
or the coordinate of reference mark associated with the land
pattern on target platform, the probe configuration, the size of
alignment mark on component to be placed, and so on, are entered
into the database of placement system. The information is useful in
the placement automation. [0117] b. Load a target platform to
assembly equipment. A reference point on target platform is chosen
as the origin to match the origin of the coordinate system stored
in placement database. [0118] c. The probe can be a single-sensor
probe for monitoring the placement accuracy, or it can be a
multiple-sensor probe for fixing the deviation of a displaced
component. To fix the deviation of component not accurately placed,
it needs to align the probe to the center of target reference mark
prior to placing component on target platform. The probe can be a
resistance probe, which measures the on/off conduction from probe
sensor, through alignment mark, to reference mark via a direct
contact approach. The probe can be a capacitance probe, which
detects the extent of area overlap between the probe and the target
reference via a non-contact approach. The probe can also be an
optical probe, which measures optical alignment between the probe
and a target reference.
[0119] Aligning probe to the center of a reference mark can be done
by moving the probe to the coordinate of reference mark per
placement database and adjusting the probe position, if necessary,
until all sensors detecting current flow are the inner sensors
around the center of the probe. For a resistance probe, the probe
must be lowered down to make a direct contact with the reference
mark. For a no-direct-contact capacitance probe, the probe needs to
move to the range that no foreign conduction object is within the
range of its electrical field. For an optical probe, the probe
needs to adjust height to have the light beam matches the diameter
of the target reference.
[0120] The inner sensors detecting current flow, based on the
measurement through reference mark, determine the range of active
sensors on probe. With proper adjustment the center of these active
sensors can match the center of reference mark automatically. The
center and the range of active sensors on probe are the new
position and target for the alignment mark on component to be
aligned to. The multiple-senor probe is able to provide feedback
for component position adjustment, which is different from a
single-sensor probe that only can detect the placement accuracy.
[0121] d. Pick up a component and place it at a target land
pattern. The placement is based on the coordinate of the center of
target land pattern or the coordinate of reference mark. [0122] e.
Adjust the component pickup header until the position of component
alignment mark is in line with the probe. The direction of
adjustment is in the direction to make more active sensors detect
current flows. The alignment is achieved when the alignment mark is
in the position that all effective active sensors on probe can
detect its presence.
[0123] Since the spatial relationship between the reference mark
and the land pattern on target platform can be pre-fabricated to
match the spatial relationship between the component's alignment
mark and its contact array, the component can then be well aligned
to the target land pattern. Some minor out-of-specification
components could be also used in assembly under such alignment
technology. [0124] f. For components with embedded indirect
alignment path, all indirect alignment paths can be connected in a
daisy chain to form a serial alignment chain or several shorter
alignment chains. The alignment chain can be used to detect the
connection status for all components in the chain when the system
is put in use
[0125] The above sequence of steps illustrates a method according
to an embodiment of the present invention. Other alternatives can
also be provided where steps are added, one or more steps are
removed, or one or more steps are provided in a different sequence
without departing from the scope of the claims herein.
[0126] The alignment technique is not limited to the placement of
components on PCBs in assembly. It can also be used in a variety of
applications, such as the monitoring of the chip packaging, where
the die to be mounted onto a substrate is viewed as the component
and the substrate of the package is viewed as the target platform.
Eventually, the alignment mark and probing technique can be applied
to multiple package stacking, the bare die stacking, the multi-chip
carrier module assembly, the module assembly, the encapsulated card
assembly, and the motherboard assembly, and so on. One of ordinary
skills in the art would recognize many variations, modifications,
and alternatives.
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